Government laws and regulations require telephone calls to be compatible with emergency 911 (E911) systems by selecting a public safety answering point (PSAP) in an area proximate to the caller's location and providing the PSAP with the caller's location. For conventional fixed-line stations in which calls are initiated over the circuit-switched, public switched telephone network (PSTN), this has conventionally been implemented using databases containing location information (e.g., a street address) associated with each telephone number.
Unlike fixed-line stations, mobile stations do not have a fixed location that can be stored in a database. Accordingly, supporting E911 for mobile stations requires determining the location of the mobile station when the emergency call is initiated. Because of the difficulties of supporting E911 for mobile stations, government regulations provide for two different types of support for E911, E911 Phase I and E911 Phase II, the difference being the granularity of the location of the mobile station. E911 Phase I provides the PSAP with the location of the radio tower (i.e., cell site tower) that supports the mobile station, which is a very coarse measure of the mobile station's location. The granularity of the location for E911 Phase II support depends upon whether the mobile station's location is a device-based technique or a network-based technique. A device-based technique requires that the location of the mobile station be within 50 meters or less 67% of the time, and the location of the mobile station be within 150 meters or less 95% of the time. A network-based technique requires that the mobile station's location be within 100 meters 67% of the time and the mobile station's location is within 300 meters 95% of the time.